1. Field of the Invention
The present invention relates to a programmed electrode wear compensation device and method for a 3D EDM scanning apparatus, particularly to a programmed electrode wear compensation device and method used in conjunction with a 3D EDM scanning apparatus, which ensures that smoothness and shape of a machined surface are maintained during electric discharge.
2. Description of Related Art
Machining by electric discharge is a working method which is slow, but applicable to working hard materials or alloys, which are usually difficult to work, and which is mainly used for generating micro-sized forms or cuts, with dimensions ranging from dozens of micrometers to several millimeters.
Referring FIGS. 10 and 11, in conventional art, wire electric discharge grinding (WEDG), structures as shown in FIGS. 10 are made by an arrangement as follows: A workpiece 71 is worked by a running wire 72 which is guided by a wire guide 73. Working is performed by arranging a micro-sized electrode and is milling layers with the electrode by electric discharge. During working, however, the electrode undergoes wear, which needs to be tracked and compensated to maintain plane surfaces and homogeneous milling depths as well as to preserve smooth surfaces. Due to working, electrodes are usually not flat and become shorter over time, so that worked surfaces are not even and milling depths are not reached. Using small electrodes that run along linear tracks to work relatively large areas without compensating for wear results in uneven worked surfaces. Therefore, proper tracking and wear compensation are indispensable in EDM scanning.
Research has established relations between material, shape, dimensions, energy of discharge, working paths and electrode wear. In practice, however, working is still performed by individual judgement of users without clear rules. Generally, electrode wear is compensated only when proceeding to another layer. Programming support for EDM is provided only by regular computer aided manufacturing (CAM) and complemented by the experience of the user.
Conventional art is summarized as follows:    1. Taiwan patent publication no. 520311 “Method for working holes with uniform depths by automatic compensation of electrode wear” teaches an automatic calculation of electrode wear, which is compensated in working subsequent holes. Although this method results in increased effectiveness of working, it is only applicable to working holes.    2. U.S. Pat. No. 4,301,349 “Electrical machining apparatus for forming a three-dimensional surface contour in a workpiece” teaches a wire oscillating at high frequency, so that continuously changing sections of the wire are used for working to eliminate effects of wear.    3. U.S. Pat. No. 4,386,248 “Electrical machining method and apparatus for forming a 3D surface contour in a workpiece with a travelling wire electrode” teaches a continuously running wire as discharge electrode, so that changing sections of the wire are used for working to eliminate effects of wear.    4. U.S. Pat. No. 5,354,961 “Device and process for electrical-discharge machining of a three-dimensional cavity with a thin rotating tool electrode” discloses a controller which calculates wear and change of shape of a tool electrode and measures and adjusts the length of the electrode. The controller calculates volume changes, which possibly are not correct. Working paths are not controlled, so that working time is wasted.    5. U.S. Pat. No. 6,454,930 “Procedure and device for the three-dimensional processing of a work piece by means of electroerosive or electrochemical machining” discloses a device moving with five degrees of freedom and working a workpiece by electroerosive or electrochemical machining. An angle between the workpiece and an electrode is adjustable, allowing for angular compensation between different layers. Working depths are small, and working and sensing are done simultaneously, which slows down working and increases manufacturing cost.    6. U.S. Pat. No. 6,531,036 “Fabrication of micron-sized parts from conductive materials by silicon electric discharge machining” teaches an electrode which is made by engraving semiconductor material and depositing a metal layer thereon and used in conjunction with CAD/CAM for working plane surfaces. Therein, a microelectronic process and electric discharge are used for CAD/CAM working.    7. U.S. Pat. No. 6,642,470 “Process and device for machining a three-dimensional piece by electroerosive milling” teaches a manufacturing process in which a regulation module compensates for electrode wear by simulation thereof and comparison with an actual electrode position. Compensation rules, however, are complicated, so that handling is difficult. Furthermore, crossing of working path sections is not taken into consideration.
Similar to conventional milling, conventional working by electric discharge is performed-using procedures and parameters that are the result of experience. Material, dimensions, roughness of surfaces and user experience decide on working parameters like waveform, voltage and computing parameters. Usually, a groove is made by clearing away material layer by layer, with plane of worked surfaces being determined by working paths and compensation of electrode wear. Optimal speed of working depends on electrode wear and is decided by experience. So far there is no CAD/CAM technique on the market that considers working paths. Any working program still has to overcome this problem.